The broad goal of this research is to characterize the nature of the internal acoustic structure of the phonetic categories of speech. The search for stable acoustic properties corresponding to phonetic features will be expanded to include an investigation of geminate stop consonants by examining the closure duration and the amplitude and spectral properties of the burst, the stop/glide manner of articulation by examining the patterns of amplitude change occurring in particular frequency regions in the vicinity of the stop/glide release, and laterals by examining the spectral and temporal properties over the course of the lateral sound segment in both consonantal and vocalic positions. The effects of particular sources of variability on the acoustic properties associated with a particular feature will be investigated including the effects of the juxtaposition of consonant properties on adjacent properties for both voicing and place of articulation, the effects of speaker-rate and the phonetic inventory of a particular language on the nature of the acoustic properties. The extent to which acoustic/phonetic properties affect lexical access will be investigated including an examination of subphonetic changes in intervocalic as well as initial position and across different word stress conditions, an examination of acoustic variation that affects the 'prototypicality' of the category, and phonetic manipulations that contribute to the nasal manner of articulation, place of articulation, and the 'goodness' of a stop consonant. The general research strategy includes acoustic analyses of speech to explore the nature of the acoustic properties, computer-editing and synthesis of speech to examine the perception of the acoustic properties of speech, and lexical decision experiments to explore the effects of acoustic/phonetic properties on lexical access. This research should help characterize the nature of the acoustic properties corresponding to the phonetic features of language, and should provide a greater understanding of the speech processing system, its underlying mechanisms, and how it interfaces with higher levels of linguistic processing. Characterizing the speech processing of normal adults serves as an essential baseline for understanding the nature of speech/language disabilities.